1. Field of the Invention
The invention relates generally to the field of seismic data processing and interpretation. More specifically, the invention relates to methods for determining the quality factor (O) from seismic data.
2. Background Art
Seismic exploration techniques are used to locate subsurface earth formations that are likely to produce economically useful materials such as petroleum. Seismic exploration techniques include deploying one or more seismic energy sources near the earth's surface and deploying an array of seismic sensors at or near the surface in the vicinity of the one or more seismic sources. Seismic energy propagates downwardly from the source, where it may be reflected by subsurface acoustic impedance boundaries. The reflected seismic energy is detected by an array of sensors. The sensors generate electrical and/or optical signals corresponding to the detected seismic energy. The signals are typically recorded for processing.
Seismic processing known in the art includes determining structures of the subsurface earth formations. Typically, structures are inferred by analysis of the two-way travel time of the seismic energy from the source to the various reflective boundaries beneath the surface and back to the sensors at or near the surface.
It is also known in the art to determine various petrophysical properties of the subsurface earth formations by analysis of the frequency content of the detected seismic energy and the phase and amplitude relationships between the seismic energy generated by the source and the seismic energy detected by the sensors. Such analysis includes determining one or more seismic “attributes” of the earth formations. Attributes may be computed prestack or poststack. Prestack means processing prior to summing or “stacking” individual sensor recordings (“traces”) according to a predetermined relationship, such as common mid point (CMP) or common depth point (CDP). Poststack refers to processing after individual sensor recordings have been summed or stacked. Poststack attributes include, for example, reflection intensity, instantaneous frequency, reflection heterogeneity, acoustic impedance, velocity, dip, depth and azimuth. Prestack attributes include moveout parameters such as amplitude-versus-offset (AVO), and interval and average velocities. Further, attributes may be categorized either as instantaneous attributes, wavelet attributes or geometrical attributes. Instantaneous attributes are attributes whose values are obtained for each data point in the seismic data or within a small time window of data points (e.g., a few milliseconds), such as amplitude, phase, frequency and power. “Data points” within seismic data typically refers to numbers each representing a value of seismic trace amplitude at the instant in time at which each of the amplitude values is recorded. Wavelet attributes are the instantaneous attributes computed at the maximum point of the envelope. The physical meaning of all the wavelet attributes is essentially the same as their instantaneous attribute counterparts. Geometrical, or interval, attributes are attributes of a seismic trace within a seismic interval. Interval attributes are computed from the reflection configuration and continuity. The following references describe aspects of seismic attributes and their applications.
U.S. Pat. No. 5,226,019 issued to Bahorich states that with reference to seismic attributes, “combining multiple (i.e. two or more) descriptors through addition, subtraction, multiplication and ratio, or other means can also be successfully employed”, and suggests the use of “a product of the average instantaneous amplitude and average instantaneous frequency.”
U.S. Pat. No. 5,884,229 issued to Matteucci, discloses a statistical method for quantitatively measuring the lateral continuity of the seismic reflection character of any specified location in a subsurface target formation.
U.S. Pat. No. 5,930,730 issued to Marfurt et al., discloses a system for forming a seismic attribute display from calculated measures of semblance and corresponding estimates of true dip and true dip azimuth of seismic traces within an analysis cell.
U.S. Pat. No. 6,012,018 issued to Hornbuckle, relates to a system for identifying volumetric subterranean regions bounded by a surface in which a specific seismic characteristic has a constant value. It is stated in the '018 patent that, “in a geological region where physical characteristics (e.g., the presence of oil or gas) are well-correlated with seismic attributes, (e.g., seismic amplitude data), the identification of a subvolume bounded by a constant-seismic-attribute-value surface may provide a very useful predictor of the volumetric extent of the attribute and hence of the characteristic.”
U.S. Pat. No. 5,001,677 issued to Masters, discloses a system, which treats measured attributes derived from seismic data as components of a vector, estimates a background vector representing typical background geologic strata, and then calculates a new attribute. As stated in the '677 patent, the preferred embodiment combines information about P (compressional) and S (shear) impedance contrasts so as to discriminate prospective reservoir strata from surrounding non-reservoir or background strata.
U.S. Pat. No. 5,724,309 issued to Higgs et al, discloses a system in which two new seismic attributes (dip magnitude and dip azimuth) are derived from instantaneous phase. The system comprises determining a spatial frequency value by taking the directional spatial derivative of the instantaneous phase for each of a plurality of x, y, t(z) data points in the seismic data and posting the spatial frequency values to identify changes within the earth's subsurface.
U.S. Pat. No. 5,870,691 issued to Partyka et al., discloses a method for processing seismic data to identify thin beds.
Although it is generally recognized that specific seismic attributes are related to specific subsurface properties, a need continues to exist for advancements in the use of seismic attributes to improve the delineation of subsurface regions of the earth to assist in the exploration and production of oil, natural gas and other minerals. There is continuing interest in methods for analyzing seismic data so as to provide direct indication of the presence of petroleum beneath the earth's surface.
Quality factor (O) is a measure of how earth formations attenuate and disperse acoustic (seismic) energy. Q has been used as a direct indicator of the presence of hydrocarbons, among other uses. Estimation of attenuation of pressure and shear waves is as important as the estimation of interval velocities in the field of seismic data interpretation. Estimates of attenuation of pressure and shear waves provide an additional perspective of the lithology (rock mineral composition) and reservoir characteristics (rock pore space fluid content, fluid composition, fluid pressure and rock permeability to fluid flow).
On the other hand, high frequency seismic energy losses due to absorption reduce the bandwidth of the seismic waves passing through the earth formations, and consequently reduce resolution of the seismic images that can be made from reflection seismic recordings. In one case it is desirable to quantify the attenuation effects and in the second case it is desirable to compensate for the attenuation. Due to the long wavelength and low frequency content of seismic waves, the attenuation effects are usually very small. They can be accurately estimated only over large depth (seismic energy travel) intervals. Measurement over shorter intervals can be noisy to such degree that the interpretations made from such estimates may be questionable. Using Q computation techniques known in the art, many Q values are computed and the results are viewed to detect a causal relationship between the lithology and the formation structures inferred from the seismic section.
There have been a number of methods proposed to calculate Q, mostly based on the power spectral ratio concept. Several of these methods are based on the seismic envelope rise time and on the normalized maximum envelope slope. Spectral ratio methods, while appearing to provide the most reliable results in Q calculation, have tended to be unreliable primarily due to near zero values in the denominator spectrum used in the spectral division step implementing the spectral ratio method.